=================
  The EFI Boot Stub
  =================

  On the x86 and ARM platforms, a kernel zImage/bzImage can masquerade
  as a PE/COFF image, thereby convincing EFI firmware loaders to load
  it as an EFI executable. The code that modifies the bzImage header,
  along with the EFI-specific entry point that the firmware loader
  jumps to are collectively known as the "EFI boot stub", and live in

  arch/x86/boot/header.S and arch/x86/boot/compressed/eboot.c,

  respectively. For ARM the EFI stub is implemented in
  arch/arm/boot/compressed/efi-header.S and
  arch/arm/boot/compressed/efi-stub.c. EFI stub code that is shared

  between architectures is in drivers/firmware/efi/libstub.

  For arm64, there is no compressed kernel support, so the Image itself
  masquerades as a PE/COFF image and the EFI stub is linked into the
  kernel. The arm64 EFI stub lives in arch/arm64/kernel/efi-entry.S

  and drivers/firmware/efi/libstub/arm64-stub.c.

  By using the EFI boot stub it's possible to boot a Linux kernel
  without the use of a conventional EFI boot loader, such as grub or
  elilo. Since the EFI boot stub performs the jobs of a boot loader, in
  a certain sense it *IS* the boot loader.
  
  The EFI boot stub is enabled with the CONFIG_EFI_STUB kernel option.

  How to install bzImage.efi
  --------------------------

  
  The bzImage located in arch/x86/boot/bzImage must be copied to the EFI

  System Partition (ESP) and renamed with the extension ".efi". Without

  the extension the EFI firmware loader will refuse to execute it. It's
  not possible to execute bzImage.efi from the usual Linux file systems

  because EFI firmware doesn't have support for them. For ARM the
  arch/arm/boot/zImage should be copied to the system partition, and it

  may not need to be renamed. Similarly for arm64, arch/arm64/boot/Image
  should be copied but not necessarily renamed.

  Passing kernel parameters from the EFI shell
  --------------------------------------------

  Arguments to the kernel can be passed after bzImage.efi, e.g.::

  
  	fs0:> bzImage.efi console=ttyS0 root=/dev/sda4

  The "initrd=" option
  --------------------

  
  Like most boot loaders, the EFI stub allows the user to specify
  multiple initrd files using the "initrd=" option. This is the only EFI
  stub-specific command line parameter, everything else is passed to the
  kernel when it boots.
  
  The path to the initrd file must be an absolute path from the
  beginning of the ESP, relative path names do not work. Also, the path
  is an EFI-style path and directory elements must be separated with

  backslashes (\). For example, given the following directory layout::

    fs0:>

  	Kernels\
  			bzImage.efi
  			initrd-large.img
  
  	Ramdisks\
  			initrd-small.img
  			initrd-medium.img
  
  to boot with the initrd-large.img file if the current working

  directory is fs0:\Kernels, the following command must be used::

  
  	fs0:\Kernels> bzImage.efi initrd=\Kernels\initrd-large.img
  
  Notice how bzImage.efi can be specified with a relative path. That's
  because the image we're executing is interpreted by the EFI shell,
  which understands relative paths, whereas the rest of the command line
  is passed to bzImage.efi.

  The "dtb=" option
  -----------------

  For the ARM and arm64 architectures, we also need to be able to provide a
  device tree to the kernel. This is done with the "dtb=" command line option,

  and is processed in the same manner as the "initrd=" option that is
  described above.